# [leetcode] Count Univalue Subtrees

### Count Univalue Subtrees

Given a binary tree, count the number of uni-value subtrees.

A Uni-value subtree means all nodes of the subtree have the same value.

For example:
Given binary tree,

```              5
/ \
1   5
/ \   \
5   5   5
```

return `4`.

```/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int countUnivalSubtrees(TreeNode* root) {
if(root == nullptr) return 0;
int res = 0;
bool isUni = true;
DFS(root, res, isUni);
return res;
}
//return uni value, res stores the num of univalue substrees.
int DFS(TreeNode * root, int & res, bool & isUni){
//leaf node
if(root->left == nullptr && root->right == nullptr){
isUni = true;
res++;
return root->val;
}
bool isLeftUni = true, isRightUni = true;
int leftval, rightval;
if(root->left){
leftval = DFS(root->left, res, isLeftUni);
}
if(root->right){
rightval = DFS(root->right, res, isRightUni);
}
if(isLeftUni && isRightUni){
leftval = root->left? leftval: root->val;
rightval = root->right? rightval: root->val;
isUni = (leftval == rightval) && (root->val == leftval);
if(isUni == true){
res++;
}
return root->val;
}
else{
//left or right is not uni
isUni = false;
return root->val;//useless
}
}
};```

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